1. Field of the Invention
The present invention relates to a Passive Optical Network (PON), and more particularly, to a Reflective Semiconductor Optical Amplifier (RSOA), an RSOA module having the RSOA, and a PON using the RSOA.
2. Description of the Related Art
Fiber To The Home (FTTH) technology for directly connecting a telephone switch to a home through optical fiber has been actively researched and developed worldwide in order to provide voice, data, and broadcast convergence services to subscribers and is expected to become popular within a few years. From the point of view of the characteristics of an optical network, it is most important to develop an optical signal transmission method having superior economical efficiency and mass productivity in developing the FTTH technology. Optical networks may be classified into a Passive Optical Network (PON) and an Active Optical Network (AON). The PON is currently being developed in the forms of an Asymmetric Transfer Mode (ATM)-PON, a B-PON, a G-PON and an E-PON. The AON is being developed into a form that connects local networks, each of which is composed of Ethernet switches, using optical fiber.
A Wavelength Division Multiplexing (WDM)-based FTTH network, that is, a WDM-PON, is a scheme in which the communication between a center office (CO) and subscribers is performed using a wavelength assigned to each subscriber. Such a WDM-PON is advantageous in that the WDM-PON can provide independent and high-capacity communication service to each subscriber, and has excellent security. Furthermore, in the WDM-PON, the modulation and demodulation of light are performed for each subscriber, unlike a Time Division Multiplexing (TDM) type, so that an optic source having low modulation speed and output and a receiver having narrow bandwidth can be employed.
However, since intrinsic wavelengths are respectively assigned to subscribers for communication with the CO, the WDM-PON has a limited wavelength band and interval, and thus, the number of subscribers is also limited. Moreover, although a transmission speed per wavelength is at least 1 Gbps, there is no content to be transmitted for which this high speed is appropriate.
From an economical point of view, the WDM-PON is more expensive since it requires a pair of optical transceiver modules for each subscriber, which are respectively installed in a subscriber's area and the CO. In addition, the WDM-PON requires optic sources having intrinsic wavelengths, the number of which is identical to the number of subscribers, so that an economical burden is imposed on subscribers and, therefore, implementation of the WDM-PON is difficult. From a maintenance point of view, the service provider is disadvantaged in that it must prepare different optic sources having different wavelengths for individual subscribers for installation and breakdowns. Accordingly, it is important to develop a low cost WDM-PON optic source and the provision of the same kind of wavelength-independent optic sources to all subscribers is necessarily required to implement the WDM-PON.
Meanwhile, a method of forming a WDM-PON that uses a Reflective Semiconductor Optical Amplifier (RSOA) and thus does not need independent seed light for an upstream signal (hereinafter referred to as a “seedless RSOA-based WDM-PON”) and its applications have been suggested. In the operation of the seedless RSOA-based WDM-PON, an optical signal modulated into a downstream data output from the CO (hereinafter, referred to as a “downstream signal”) is input to an RSOA at an optical network terminal (ONT) and the RSOA is operated in a gain-saturation region with respect to input optical signal power so that a difference between levels (i.e., level 0 and level 1) of the input optical signal is remarkably squeezed and the optical signal is remodulated into upstream data (hereinafter, referred to as an “upstream signal) and is transmitted to the CO.
In an RSOA structure, a usual SOA chip has a backside with a high-reflective (HR) coating facet and a front side with an anti-reflective (AR) coating facet. Light incident onto the front side is amplified while progressing in an active waveguide, is then reflected from the HR coating facet on the backside, and is then output through the front side. The RSOA structure may be classified into a weakly-index guided structure and a strongly-index guided structure according to the structure of a waveguide in the active region. The weakly-index guided structures include many kinds of waveguide structures, but a ridge waveguide structure is a typical structure. The strongly-index guided structure includes a planar buried heterostructure and a stripe buried heterostructure.
FIGS. 1A and 1B are horizontal and vertical cross-sections of a conventional RSOA 10 into which a passive spot-size converter is integrated. Referring to FIG. 1A, the RSOA 10 includes an active region 11, a p-type electrode 13 supplying current to the active region 11, and a passive spot-size converter 12. An HR coating facet 14a and an AR coating facet 14b are formed on both ends, respectively, of the RSOA 10. The AR coating facet 14b is an exit surface 14c for output light.
The passive spot-size converter 12 is formed using a material different from that of the active region 11 and is connected with the active region 11 using a butt-joint method. The passive spot-size converter 12 is inclined at a predetermined angle of θ with respect to a normal A of the exit surface 14c in order to improve an AR property. The predetermined angle of θ is in a range of 0 through 10 degrees.
Referring to FIG. 1B, an n-type electrode 15 is formed at the bottom of the RSOA 10. Accordingly, the current supplied by the p-type electrode 13 flows across the active region 11 to generate light and then flows into the n-type electrode 15.
The passive spot-size converter 12 increases the size of an optical mode generated to an appropriate range, thereby increasing the optical coupling efficiency between the RSOA 10 and optical fiber or other waveguides. A method of gradually decreasing an initial width Wactive of the active region 11 to an end width Wtaper of the passive spot-size converter 12, a method of gradually decreasing an initial thickness Hactive of the active region 11 to an end thickness Htaper of the passive spot-size converter 12, or a combination of them may be used to increase the size of the optical mode. Usually, the initial width Wactive is 0.8-1.5 μm; the initial thickness Hactive is 0.1-0.4 μm; the end width Wtaper is 0.1-0.5 μm; and the end thickness Htaper is 0.01-0.05 μm.
The performance of the passive spot-size converter 12 is usually evaluated based on a far-field angle. It is preferable that the far-field angle be 25 degrees or less in both of vertical and horizontal directions in order to increase the optical coupling efficiency between the passive spot-size converter 12 and optical fiber or other waveguides.
A length Lchip of the RSOA 10 may be 600-1500 μm but may be 1000 μm or less in order to realize a cheap TO-package. The active region 11 of the RSOA 10 may have a length Lactive of 200-600 μm.
FIG. 2 is a cross-section of the RSOA 10, taken along line I-I illustrated in FIG. 1A, and particularly, is a cross-section of the RSOA 10 having a planar buried heterostructure. Referring to FIG. 2, the RSOA 10 includes the active region 11 on an n-type substrate 16, a clad 18 including a lower clad 18a and an upper clad 18b respectively disposed below and above the active region 11, and the p-type electrode 13 and the n-type electrode 15 for supplying current to the active region 11. A planar buried heterostructure 11a is disposed on the top and bottom surfaces of the active region 11. The active region 11 and the planar buried heterostructure 11a form a waveguide. In addition, a current blocking layer 17 having a two-layer structure with a p-doped layer and an n-doped layer is disposed on the left and right sides of the active region 11 so that current from the p-type electrode 13 is supplied only to the active region 11.
Meanwhile, an ohmic contact layer 19a is disposed between the upper clad 18b and the p-type electrode 13, whereby an ohmic resistance between the upper clad 18b and the p-type electrode 13 is reduced. A passivation layer 19b may be disposed on a top surface of the ohmic contact layer 19a. 
FIGS. 3A and 3B are horizontal and vertical cross-sections of an RSOA 10a into which a conventional active spot-size converter 12a is integrated. The RSOA 10a including the active spot-size converter 12a is almost the same as the RSOA 10 including the passive spot-size converter 12, with the exception of the form of an electrode and used materials. In detail, the active spot-size converter 12a is formed using the same material as the active region 11. Accordingly, the active spot-size converter 12a is formed by slanting the material of the active region 11 at an angle of θ with respect to a normal of the exit surface 14c without a butt-joint. Meanwhile, a p-type electrode 13a is formed for the active spot-size converter 12a. Other elements like the shape, the size and the far-field angle of the active spot-size converter 12a are the same as those of the passive spot-size converter 12.
The RSOA structure may be classified into a multiple quantum well structure and a bulk structure. Usually, an active region in an RSOA has an indium gallium arsenide phosphide (InGaAsP) bulk structure because it is not easy to reduce polarization-dependent gain (PDG) by controlling stress at a quantum well region in the multiple quantum well structure.
However, when the active region has the bulk structure, gain and saturation performance rapidly decreases when the temperature of the RSOA increases. Accordingly, a special temperature stabilizer is needed to use the bulk structure in a temperature range of 0-40 degrees or −20-60 degrees where communication systems are usually used. For this reason, the price of RSOA modules increases and the reliability thereof decreases. The active region may have the multiple quantum well structure to avoid these problems. However, as described above, when the multiple quantum well structure is used, it is necessary to precisely control polarization of light input to an RSOA by an optical link or to use unpolarized light due to a PDG problem.
Briefly, the multiple quantum well structure has excellent gain, saturation and temperature characteristics but has a PDG problem, while the bulk structure can reduce PDG when a tensile stress is appropriately applied during the growth of an active region but has poor gain, saturation and temperature characteristics.
Conventional methods of manufacturing an optical transceiver module or RSOA module using the above-described RSOA may be largely classified into an active alignment method and a passive alignment method. The RSOA installed within a TO-package or TO-CAN is aligned with optical fiber using the active alignment method and then fixed and packaged using laser welding. When the passive alignment method is used, the RSOA is aligned with an optical axis of a waveguide on a planar lightwave circuit (PLC) platform, aligned with optical fiber using a V-groove formed in the PLC platform, and fixed and packaged using an ultraviolet-curable or thermosetting epoxy. The passive alignment method enables easier alignment than the active alignment method, thereby being suitable for mass production, but is disadvantageous in that it needs expensive flip-chip bonding equipment and precise fabrication of a V-groove.